Maria and Paul, a couple in their forties, travel through Spain with a new friend, Claire (Romy Schneider), a younger woman. The couple's daughter is also part of the trip. On their way to Madrid, they stop in a small town and are told by police that a local man who has killed his wife and her lover is on the loose in the area. There is a massive thunderstorm and the group has no choice but to stay in the town's only hotel, which is jammed beyond capacity with other travelers in the same situation. Maria, an out of control alcoholic who lives most of her life in a walking, booze-fueled dream state, seems to be subtly encouraging her husband Paul to have a sexual relationship with Claire as a way to reintroduce sexuality into their situation. While the thunderstorm is still raging, Maria secretly steps out of the crowded hotel onto a low balcony to down a bottle of booze and accidentally sees Paul and Claire kissing on a higher balcony. She is simultaneously excited and devastated. While out in the rain Maria discovers the fugitive hiding on a nearby rooftop and decides to help him escape. She then hides the murderer, a young man, in an outcropping of rocks out in lonely stretch of highway in the desert.
She eventually tells Paul and Claire about the man, but when they go out to the desert they find that the fugitive has committed suicide with the same gun he'd killed his cheating wife with. Maria, Paul and Claire decide to keep quiet and simply travel on to Madrid as planned. While in Madrid, Maria tells Paul and Claire that she’d had hopes of adding the fugitive to the odd situation as a "fourth player" in the game, but that her true desire was to sleep with Paul once again. Maria drinks herself unconscious and scenes of Paul and Claire making love were shown while we hear Maria’s voice saying how she’d been envisioning the pair making love ever since she’d first seen Claire naked. It is never made clear whether this sequence is a dream or something Maria actually observed while massively drunk. But the next day she tells Paul about it and it seems to excite him. They start to make love, but she can’t. She tells him she doesn’t love him anymore. He tells her he doesn’t believe her. Later, the trio goes to a club to watch flamenco dancers. Maria, who seems to be having a good time, slips out during the performance and disappears into the city. Paul and Claire search for her, but they cannot find her.
Melina Mercouri as Maria
Romy Schneider as Claire
Peter Finch as Paul
Julián Mateos as Rodrigo Paestra
Isabel María Pérez as Judith
Beatriz Savón as Rodrigo's Wife
10:30 P.M. Summer was released to DVD by MGM Home Entertainment on July 24, 2007 as a Region 1 widescreen DVD.